1. Field of the Invention
The present invention relates to a method for producing a functional film which comprises at least a functional layer comprising a compressed layer of functional fine particles on a support. In the present invention, the functional film includes both a functional film and a functional sheet. In addition, the functional film of the present invention includes a functional film in which a support is a metal.
The functional layer is a layer having a function, and the function means an action accomplished through physical and/or chemical phenomena. The functional layer includes layers having various functions, such as a conductive layer, an ultraviolet shielding layer, an infrared shielding layer, a magnetic layer, a ferromagnetic layer, a dielectric layer, a ferroelectric layer, an electrochromic layer, an electroluminescent layer, an insulating layer, a light-absorbing layer, a light selecting absorbing layer, a reflecting layer, a reflection preventing layer, a catalyst layer, a photocatalyst layer and others.
Particularly, the present invention relates to a method for producing a transparent conductive film having a transparent conductive layer. The transparent conductive layer can be used as a transparent electrode such as an electroluminescence panel electrode, an electrochromic element electrode, a liquid crystal electrode, a transparent plane heater, or a touch panel, and can be also used as a transparent electromagnetic-wave shielding layer.
2. Disclosure of the Related Art
Hitherto, functional films made of various functional materials are produced by the physical vapor deposition method (PVD) such as vacuum vapor deposition, laser ablation, sputtering, or ion plating, or by the chemical vapor deposition method (CVD) such as heat CVD, light CVD, or plasma CVD. These generally require a large-scale apparatus, and among these, some are not suited for forming a film of large area.
Also, a process is known in which a film is formed by application using the sol-gel method. The sol-gel method is suited for forming a film of large area, but in most cases, an inorganic material must be sintered at a high temperature after the application.
For example, with respect to transparent conductive films, the following description can be made. At present, transparent conductive film is produced mainly by the sputtering method. There are various modes for the sputtering method, for example, a method of forming a film by allowing inert gas ions, which are generated by direct current or high-frequency discharge, to be accelerated to hit the surface of a target in vacuum so as to strike out atoms constituting the target from the surface for deposition on the support surface.
The sputtering method is excellent in that a conductive layer having a low surface electric resistance can be formed even if it has a large area to some extent. However, it has a disadvantage that the apparatus is large, and the film forming speed is slow. If the conductive layer is to have a still larger area from now on, the apparatus will be further enlarged. This raises a technical problem such that the controlling precision must be heightened and, from another point of view, raises a problem of increase in the production cost. Further, although the number of targets is increased to raise the speed in order to compensate for the slowness of the film forming speed, this also is a factor that enlarges the apparatus, thereby raising a problem.
An attempt is made to produce a transparent conductive layer by the application method. In a conventional application method, a conductive paint having conductive fine particles dispersed in a binder solution is applied onto a support, dried, and hardened to form a conductive layer. The application method has an advantage in that a conductive layer having a large area can be easily formed, that the apparatus is simple and has a high productivity, and that the conductive layer can be produced at a lower cost than by the sputtering method. In the application method, an electric path is formed by contact of the conductive fine particles with each other, whereby the electric conductivity is exhibited. However, the conductive layer produced by the conventional application method has an insufficient contact, and the obtained conductive layer has a high electric resistance value (i.e. is inferior in conductivity), thereby limiting its usage.
As the production of a transparent conductive layer by a conventional application method, Japanese Laid-open Patent Publication No. 9-109259 (1997) discloses a production method comprising the first step of applying a paint comprising a conductive powder and a binder resin onto a plastic film for transcription and drying it to form a conductive layer, the second step of pressing (5 to 100 kg/cm2) the conductive layer surface on a smooth surface and heating (70 to 180° C.), and the third step of laminating this conductive layer on a plastic film or sheet and heat-press-bonding it.
In this method, a large amount of binder resin is used (100 to 500 parts by weight of conductive powder with respect to 100 parts by weight of binder in the case of inorganic conductive powder; 0.1 to 30 parts by weight of conductive powder with respect to 100 parts by weight of binder in the case of organic conductive powder), so that a transparent conductive film having a low electric resistance value cannot be obtained.
For example, Japanese Laid-open Patent Publication No. 8-199096 (1996) discloses a method in which a conductive layer forming paint comprising a tin-doped indium oxide (ITO) powder, a solvent, a coupling agent and an organic or inorganic acid salt of metal, and not containing a binder is applied onto a glass plate and calcined at a temperature higher than 300° C. In this method, since a binder is not used, the conductive layer will have a low electric resistance value. However, since a calcining step at a temperature higher than 300° C. must be carried out, it is difficult to form a conductive layer on a support such as a resin film. In other words, the resin film will be melted, carbonized, or burnt by the high temperature. Although it depends on the type of the resin film, the temperature of 130° C. may be a limit in the case of polyethylene terephthalate (PET) film, for example.
Japanese Patent Publication No. 2994764 (B2)(1999) discloses a production method of a transparent conductive film, wherein a paste made in such a way that super-fine particle powder of ITO is dispersed in a solvent together with a resin is applied onto a resin film, and is subjected to a rolling process by a steel roller, after drying.
Japanese Laid-open Patent Publication No. 7-235220 (1995) discloses a method comprising the steps of applying a dispersion liquid, which contains conductive fine particles such as ITO and which does not contain a binder, onto a glass support; slowly drying the dispersion liquid; applying an overcoat liquid made of silica sol onto the obtained ITO film; and then drying or calcining after drying. According to the aforesaid Publication, the overcoat film made of silica sol is dried for hardening and shrinking, and the ITO fine particles in the ITO film are brought into firm contact with each other by a hardening-shrinking stress at that time. If the contact between the ITO fine particles is insufficient, the electric resistance of the conductive film is high. In order to obtain a large hardening-shrinking stress, the overcoat film must be subjected to a drying process at a high temperature of 150 to 180° C. However, when the support is a resin film, the resin film will be deformed by such a high temperature.
Also, according to the aforesaid Publication, the overcoat made of silica sol contributes to bonding of the conductive film and the glass support as well. Namely, the strength of the conductive film is obtained by the overcoat made of silica sol. However, the electric resistance of the conductive film is high and the strength of the film is small unless the application and the hardening-shrinking of the overcoat liquid is carried out. Furthermore, in order to improve the optical characteristics of the conductive film and to reduce the surface resistance, the drying step after application of the dispersion liquid of the conductive fine particles on the glass support must be carried out slowly. There is a disadvantage in that cracks may be generated in the overcoat film made of silica sol if the thickness of the film is large.